Engine brakes or retarders are used to assist and supplement wheel brakes in slowing heavy vehicles, such as tractor-trailers. Engine brakes are desirable because they help alleviate wheel brake overheating. As vehicle design and technology have advanced, the hauling capacity of tractor-trailers has increased, while at the same time rolling resistance and wind resistance have decreased. Thus, there is a need for advanced engine braking systems in today's heavy vehicles.
Problems with existing engine braking systems include high noise levels and a lack of smooth operation at some braking levels resulting from the use of less than all of the engine cylinders in a compression braking scheme. To maximize fuel economy, tractor-trailers are typically operated at a relatively low engine speed, i.e. 1300 RPM. Existing braking systems are only marginally effective at such low engine speeds and often the driver must downshift to obtain acceptable engine braking performance. Also, existing systems are not readily adaptable to differing road and vehicle conditions. Still further, existing systems are complex and expensive.
Known engine compression brakes convert an internal combustion engine from a power generating unit into a power consuming air compressor.
One type of engine compression braking system utilizes an exhaust brake valve which is disposed within the exhaust pipe of an internal combustion engine. Such a system is disclosed in U.S. Pat. No. 4,054,156 issued to Benson on Oct. 18, 1977. The exhaust brake valve increases back pressure in the exhaust system by restricting the flow of exhaust in the exhaust pipe, and thereby increases the amount of work required to rotate the engine.
U.S. Pat. No. 3,220,392 issued to Cummins on Nov. 30, 1965, discloses an engine braking system in which an exhaust valve located in a cylinder is opened when the piston in the cylinder nears the top dead center (TDC) position on the compression stroke. An actuator includes a master piston, driven by a cam and pushrod, which in turn drives a slave piston to open the exhaust valve during engine braking. The braking that can be accomplished by the Cummins device is limited because the timing and duration of the opening of the exhaust valve is dictated by the geometry of the cam which drives the master piston and hence these parameters cannot be independently controlled.
U.S. Pat. No. 3,234,923 issued to Fleck et al. on Feb. 15, 1966, discloses a mechanically driven engine braking system which selectively advances the timing of the opening of exhaust valves of the engine when the engine is in a braking mode. This timing change is accomplished by rotating the exhaust camshaft of the engine with respect to the crankshaft when engine braking is desired. This effectively converts the engine from a four cycle mode to a two cycle mode wherein blow-down and intake occur during each revolution of the crankshaft.
U.S. Pat. No. 4,150,640 issued to Egan on Apr. 24, 1979, discloses an engine braking system which uses a fuel injector rocker arm to drive an hydraulic actuator which opens a pair of exhaust valves associated with a combustion chamber near the end of the compression stroke of the piston. A pressure regulating valve is used to limit the force applied to the exhaust valves by the actuator in order to ensure that the exhaust valves are not subjected to excessive loads due to the force applied by the actuator and pressure forces in the combustion chamber. The pressure regulating valve delays opening of the exhaust valves by the actuator until the level of pressure in the combustion chamber is below a level at which the exhaust valves would be subjected to excessive loading.
U.S. Pat. No. 4,981,119 issued to Neitz et al. on Jan. 1, 1991, discloses a method of two cycle compression braking in which the exhaust valve is opened at the beginning and the end of the compression stroke, and at the beginning and the end of the exhaust stroke. Pressure is maintained in the exhaust manifold by a butterfly valve-type damper disposed in the exhaust pipe or manifold. Compared to a method in which the exhaust valve is opened at the end of the compression and exhaust stroke, the method of Neitz '119 increases the initial pressure within the engine cylinder at the beginning of the compression and exhaust strokes, thereby increasing the braking power of the engine.
U.S. Pat. No. 4,741,307 issued to Meneely on May 3, 1988, discloses a method and apparatus for braking a six cylinder engine in which a first exhaust valve associated with a first cylinder near TDC on the compression stroke is opened simultaneously with that of a second exhaust valve associated with a second cylinder near bottom dead center (BDC) on the intake stroke. In addition, a third exhaust valve associated with a third cylinder near BDC on the exhaust stroke is opened, as it would be under normal operating conditions. The method and apparatus disclosed in Meneely '307 simultaneously opens each exhaust valve associated with a set of three cylinders whenever any one of the cylinders in the set is near TDC on the compression stroke.
In conjunction with the increasingly widespread use of electronic controls in engine systems, engine braking systems have been developed which are electronically controlled by a central engine control unit.
For example, U.S. Pat. No. 5,121,324 issued to Rini et al. on Jun. 9, 1992, discloses the use of an electronic fuel injection control module which includes output signals which activate and deactivate an engine braking system when appropriate. The control module prevents the engine brake from being activated when fuel is being injected into the engine.
U.S. Pat. No. 5,121,723 issued to Stepper et al. on Jun. 16, 1992, discloses an electronic control unit which activates an engine brake only when inputs from various sensors indicate that conditions are appropriate for the activation of the engine brake.
U.S. Pat. No. 5,117,790 issued to Clarke et al. on Jun. 2, 1992, and assigned to the assignee of the present application, discloses a control system and a method for controlling the operation of an internal combustion engine in a number of modes. The control system is capable of controlling fuel injection timing and quantity, and inlet and exhaust valve opening and closing independently for each engine cylinder. The control system is also capable of operating the engine in either a four cycle braking mode or a two cycle braking mode.
U.S. Pat. No. 4,664,070 issued to Meistrick et al. on May 12, 1987, discloses an electronically controlled hydromechanical overhead apparatus which is capable of opening and closing exhaust and intake valves without utilizing a rocker arm mechanism. The overhead apparatus is capable of operating the exhaust and intake valves in a two-cycle retarding mode.
U.S. Pat. No. 5,088,348 issued to Hiramuki on Feb. 18, 1992, discloses an engine braking system used in conjunction with an automatic transmission. The electronic controller ensures that the engine brake is deactivated when the automatic transmission is shifting gears.
U.S. Pat. No. 5,086,738 issued to Kubis et al. on Feb. 11, 1992, also discloses the use of an electronic controller to activate and deactivate an engine brake. The electronic controller selectively energizes a solenoid valve which places an exhaust valve in mechanical communication with an exhaust cam which includes a secondary raised portion to open the exhaust valve at the appropriate time during engine braking. When the engine brake is not operating, the electronic controller is not energized and the movement of the exhaust pushrod and rocker arm due to the secondary raised portion of the exhaust cam is taken up by a gap or lash between the exhaust rocker arm and the exhaust valve.
Even more sophisticated systems use electronic control not only to activate and deactivate an engine braking system, but also to optimize the performance of the engine braking system.
U.S. Pat. No. 5,012,778 issued to Pitzi on May 7, 1991, discloses an engine braking system which includes a solenoid actuated servo valve hydraulically linked to an exhaust valve actuator. Hydraulic pressure (on the order of 3000 psi) is supplied by a high pressure hydraulic pump which supplies a high pressure plenum. A pressure regulator disposed between the high pressure hydraulic pump and the high pressure plenum maintains operating hydraulic pressure below a desired limit.
The servo valve disclosed in Pitzi '778 includes a high pressure source duct leading from the high pressure plenum, an actuator duct leading from the servo valve to the exhaust valve actuator and a drain duct. The servo valve has two operating positions. In a first or closed position, the high pressure duct is blocked and the actuator duct is in fluid communication with the drain duct. In this first position, pressure in the exhaust valve actuator is relieved through the drain duct to place the exhaust valve actuator in a rest position out of contact with the exhaust valve. In a second or open position, the drain duct is blocked and the high pressure duct is in fluid communication with the exhaust valve actuator.
The exhaust valve actuator disclosed in Pitzi '778 comprises a piston which, when subjected to sufficient hydraulic pressure, is driven into contact with a contact plate attached to an exhaust valve stem, thereby opening the exhaust valve. An electronic controller activates the solenoid of the servo valve. A group of switches are connected in series to the controller and the controller also receives inputs from a crankshaft position sensor and an engine speed sensor.
U.S. Pat. No. 5,255,650 issued to Faletti et al. on Oct. 26, 1993, and assigned to the assignee of the present application, discloses an electronic control system which is programmed to operate the intake valves, exhaust valves, and fuel injectors of an engine according to two predetermined logic patterns. According to a first logic pattern, the exhaust valves remain closed during each compression stroke. According to a second logic pattern, the exhaust valves are opened as the piston nears the TDC position during each compression stroke. The opening position, closing position, and the valve lift are all controlled independently of the position of the engine crankshaft.
U.S. Pat. No. 4,572,114 issued to Sickler on Feb. 25, 1986, discloses an electronically controlled engine braking system. A pushtube of the engine reciprocates a rocker arm and a master piston so that pressurized fluid is delivered and stored in a high pressure accumulator. For each engine cylinder, a three-way solenoid valve is operable by an electronic controller to selectively couple the accumulator to a slave bore having a slave piston disposed therein. The slave piston is responsive to the admittance of the pressurized fluid from the accumulator into the slave bore to move an exhaust valve crosshead and thereby open a pair of exhaust valves. The use of an electronic controller allows braking performance to be maximized independent of restraints resulting from mechanical limitations. Thus, the valve timing may be varied as a function of engine speed to optimize the retarding horsepower developed by the engine.
Electrically controlled hydraulic devices are known in the art which are capable of opening and closing engine intake and exhaust valves. For example, U.S. Pat. No. 5,224,683 issued to Richeson on Jul. 6, 1993, discloses an electrically controlled hydraulic actuator comprising a magnetically actuated pilot valve which selectively supplies hydraulic pressure to open an exhaust or intake valve of an engine. The position of the pilot valve is controlled by signals from a central engine computer.
U.S. Pat. No. 5,248,123 issued to Richeson et al. on Sep. 28, 1993, discloses an electronically controlled hydraulic valve actuator having a pilot valve which is electrically controlled via a solenoid, an intermediate valve which is moveable to supply fluid to the exhaust or intake valve of the engine, and an initializer valve which decelerates the exhaust or intake valve as it opens.
U.S. Pat. No. 4,974,495 issued to Richeson, Jr. on Dec. 4, 1990, discloses an electrically controlled hydraulically powered valve actuator capable of actuating an intake or exhaust valve of an internal combustion engine. The valve actuator uses magnetic latching to retain the valve actuator in one of two stable positions.
U.S. Pat. No. 5,022,358 issued to Richeson on Jun. 11, 1991, discloses a valve similar to the Richeson, Jr. '495 valve which also includes the capability to store the energy produced when the valve actuator opens the exhaust or intake valve. This energy is used to close the exhaust or intake valve.
U.S. Pat. No. 5,022,359 issued to Erickson et al. on Jun. 11, 1991, and U.S. Pat. No. 5,029,516 issued to Erickson et al. on Jul. 9, 1991, disclose electronically controlled actuator valves which may be used to open and close intake and exhaust valves of an internal combustion engine. The advantageous characteristics of these electronically controlled actuator valves include their fast acting capability, the fact that they can be used instead of a cam driven actuator valve and that they provide a desired flexibility in valve control during the engine braking mode. The elimination of a camshaft simplifies the engine and increases reliability due to the reduction in moving parts.
It is desired to provide an economical engine compression braking system providing increased braking performance and reliable operation over extended operating conditions.